The hydrodealkylation of alkylaromatic hydrocarbons has been practiced for many years. The principal process involves the conversion of toluene and similar alkyl substituted benezenes to benzene. Such processes may be catalytic or non-catalytic in nature. Most commercial catalytic processes employ chromia or magnesia deposited on an alumina base as the catalyst. Such catalysts for the hydrodealkylation of alkylaromatic feedstocks, for example the production of benzene from pyrolysis gasoline, tend to deactivate with use, presumably due to coke deposition on the catalyst surface. Therefore, measuring the amount of coke and the rate at which it is deposited provides a means for determining the deactivation of the catalyst.
In addition, commercial catalytic hydrodealkylation processes may use a chromia or alumina catalyst because, unlike noble metal catalysts, this catalyst is not readily susceptible to sulfur poisoning; rather it acts as a hydrodesulfurization catalyst. This aspect is advantageous since thiophene which is usually present in alkylaromatic feed streams is difficult to separate from benzene.
It is believed that acid sites promote polymerization of either hydrogenolysis products or aromatic hydrocarbons resulting in hydrocarbon condensation on the catalyst surface. Under the conditions of the process these condensed species are dehydrogenated forming coke. The result of these reactions is a reduction in activity of the catalyst since the coke is strongly absorbed onto the sites which promote dealkylation. In other words, this coke or carbon build-up either blocks or poisons the active catalyst sites causing deactivation.
Since a chromia or alumina catalyst suffers gradual deactivation due to coking under hydrodealkylation conditions, the reactor temperature must gradually be increased to maintain an acceptable level of conversion. Typically, when the conversion level drops below about 50% at 1200.degree. F. (649.degree. C.), the catalyst is regenerated. The normal cycle life may be four to six months with high coking feedstocks such as pyrolysis gasoline. Initial catalyst activity decreases with regeneration until eventually the catalyst can no longer be used due to either low activity or thiophene breakthrough.
Therefore, a catalyst which is more deactivation-resistant, or demonstrates a decreased rate of coke formation, would permit greater efficiency in the hydrodealkylation process by converting toluene to benzene at a greater rate for a longer period of time before the catalyst must be regenerated. In addition, the catalyst still must provide commercially acceptable conversion rates at an acceptable selectivity.
U.S. Pat. No. 2,692,293 discloses the selective dealkylation of alkyl substituted aromatic hydrocarbons to lower molecular weight aromatic hydrocarbons using a supported catalyst which comprises an inactive carrier having a surface area generally in excess of 50 m.sup.2 /g and containing less than about 15 wt% of a catalytic dehydrogenative material such as the metals of Group VIa including molybdenum and chromium oxides, and noble metals of the platinum-palladium group. The inactivation of the catalyst carrier material is effected by introducing into the carrier material an alkaline earth compound or an alkali metal oxide at about 0.1 to 2 wt% based on the carrier. Such deactivation is said to minimize the occurrence of side reactions which tend to produce coke and other products.
U.S. Pat. No. 2,773,917 discloses demethylation of metyl-substituted benzenes with a catalyst comprising chromia of molybdena composited with a suitable carrier. Although 4 to 12 wt% chromia supported on an alumina catalyst is suggested, only a co-precipitated chromia-alumina catalyst is shown in the examples. It is stated that maximum yield with a minimum of coke formation can be achieved by varying the reaction conditions within specified ranges.
U.S. Pat. No. 2,951,886 discloses the recovery of sulfur-free nitration grade benzene from crude coke oven or coal tar light oils by dealkylating in the presence of hydrogen at temperatures above 1200.degree. F. in the presence of a catalyst consisting of approximately 10 through 15 wt% chromium oxide on a high purity, low sodium content, gamma type alumina support. The process is said to proceed with little or no coking affect.
U.S. Pat. No. 3,277,197 discloses a hydrodealkylation process employing oxides or sulfides of the metals of Group VIb supported on alumina of high purity, preferably in the eta phase, characterized by an elevated porosity, a surface area of about 150 to 200 m.sup.2 /g and an average diameter of the pores of more than 150 Angstroms and less than 550 Angstroms.
U.S. Pat. No. 3,760,023 discloses a process for the hydrodealkylation of alkyl substituted aromatic hydrocarbons with a catalyst comprising a metal of Group VIb in an amount of about 5 to 15 wt% of the finished catalyst and 1 to 10 wt% of a promoter selected from the group consisting of alkali metals, alkaline earth metals and rare earth metals. The active metal and the promoter are deposited on an inert oxide support which preferably includes a high area alumina having a boehmite, bayerite, beta, or eta crystalline form, or other aluminas, silica-alumina, silica, silica-magnesia, silica-zirconia, alumina-magnesia, etc.
U.S. Pat. No. 3,900,430 discloses a process for converting hydrocarbon oils to desirable components by contacting the oils in the presence of hydrogen under hydrocarbon conversion conditions with a catalyst comprising a catalytic amount of a catalytic material supported on gamma alumina prepared by a specific process and having a surface area from about 225 to about 400 m.sup.2 /g. Different groupings of catalytic metals are disclosed for reforming light hydrocarbon stocks to produce gasoline, benzene and the like, for hydrosulfurizing hydrocarbon oils, and for dehydrogenating hydrocarbon oils using the described alumina as the support.
U.S. Pat. No. 3,992,468 in comparative Example 1B shows a hydrodealkylation process using a conventional hydrodealkylation catalyst containing 7.5% of chromium oxide deposited on alumina having a specific surface of 170 m.sup.2 /g, a pore volume of 0.60 cc/g.